Re: [PATCH v3 2/2] cpuhp: Expedite RCU grace periods during SMT operations

From: Vishal Chourasia

Date: Fri Mar 20 2026 - 14:50:21 EST

Hi Paul, Thank you for your response.
Sorry, I could not revert back quicker.

As I wanted to understand what's happening behind the scenes after the
cpuhp kthread blocks upon execution of synchronize_rcu(). So I did a
little more digging.

On a 320 CPU system, (SMT8 to SMT4) operation takes >1 minute to complete.
160 CPUs are offlined one by one.

In total 321 synchronize_rcu() calls are invoked taking ~125ms to finish
(ftrace option sleep-time set).

3298110.851011 | 316) cpuhp/3-1614 | | synchronize_rcu() {
3298111.010125 | 316) cpuhp/3-1614 | @ 159112.9 us | }
--
3298111.020432 | 0) kworker-29406 | | synchronize_rcu() {
3298111.190132 | 0) kworker-29406 | @ 169699.4 us | }
--
3298111.191327 | 317) cpuhp/3-1619 | | synchronize_rcu() {
3298111.350129 | 317) cpuhp/3-1619 | @ 158801.9 us | }
--
3298111.360263 | 0) kworker-29406 | | synchronize_rcu() {
3298111.530137 | 0) kworker-29406 | @ 169874.5 us | }
--
3298111.531098 | 318) cpuhp/3-1624 | | synchronize_rcu() {
3298111.650128 | 318) cpuhp/3-1624 | @ 119029.8 us | }

Breakdown of the time spent during a single synchronize_rcu() during the
invocation of sched_cpu_deactivate callback (CPU 4 was offlined)

Summary:
--> cpuhp_enter (sched_cpu_deactivate)
CB registration → AccWaitCB ~10ms Waiting for softirq tick on CPU 4
GP 220685125: FQS scan 1 ~10ms Tick delay + scan (all clear except CPU 260,
rcu_gp_kthread is running on CPU 260)
GP 220685125: wait for CPU 260 ~30msi FQS sleep interval, CPU 260 not yet reported
GP 220685125: FQS scan 2 + end ~0.02ms CPU 260 clears
GP 220685129: FQS scan 1 ~30ms Tick delay + full scan (same: CPU 260 holdout)
GP 220685129: wait for CPU 260 ~30ms Same pattern
GP 220685129: FQS scan 2 + end ~0.02ms CPU 260 clears
CB invocation + wakeup ~10ms Softirq tick invokes wakeme_after_rcu
destroy_sched_domains_rcu queueing ~8ms322 call_rcu() callbacks
<-- cpuhp_exit (sched_cpu_deactivate)

I have collected some rcu static tracepoint data, which I am currently
going through.

On Fri, Mar 06, 2026 at 07:12:04AM -0800, Paul E. McKenney wrote:
> On Fri, Mar 06, 2026 at 11:14:13AM +0530, Vishal Chourasia wrote:
> > On Mon, Mar 02, 2026 at 05:17:16PM +0530, Samir M wrote:
> > >
> > > On 27/02/26 6:43 am, Joel Fernandes wrote:
> > > > On Wed, Feb 18, 2026 at 02:09:18PM +0530, Vishal Chourasia wrote:
> > > > > Expedite synchronize_rcu during the SMT mode switch operation when
> > > > > initiated via /sys/devices/system/cpu/smt/control interface
> > > > >
> > > > After the locking related changes in patch 1, is expediting still required? I
> > Yes.
> > > > am just a bit concerned that we are papering over the real issue of over
> > > > usage of synchronize_rcu() (which IIRC we discussed in earlier versions of
> > > > the patches that reducing the number of lock acquire/release was supposed to
> > > > help.)
> > At present, I am not sure about the underlying issue. So far what I have
> > found is when synchronize_rcu() is invoked, it marks the start of a new
> > grace period number, say A. Thread invoking synchronize_rcu() blocks
> > until all CPUs have reported QS for GP "A". There is a rcu grace period
> > kthread that runs periodically looping over a CPU list to figure out all
> > CPUs have reported QS. In the trace, I find some CPUs reporting QS for
> > sequence number way back in the past for ex. A - N where N is > 10.
>
> This can happen when a CPU goes idle for multiple grace periods, then
> wakes up in the middle of a later grace period. This is (or at least is
> supposed to be) harmless because a quiescent state was reported on that
> CPU's behalf when RCU noticed that it was idle. The report is quashed
If it is harmless, can we consider just expediting the smt mode switch
operation via smt/control file [1].

Thanks, vishalc

[1] https://lore.kernel.org/all/20260218083915.660252-6-vishalc@xxxxxxxxxxxxx/

> when RCU notices that the quiescent state being reported is for a grace
> period that has already completed. Grace-period counter wrap is handled
> by the infamous ->gpwrap field in the rcu_data structure.

>
> I have seen N having four digits, with deep embedded devices being most
> likely to have extremely large values of N.
>
> Thanx, Paul
>
> > > > Could you provide more justification of why expediting these sections is
> > > > required if the locking concerns were addressed? It would be great if you can
> > > > provide performance numbers with only the first patch and without the second
> > > > patch. That way we can quantify this patch.
> > > >
> > > >
> > > SMT Mode | Without Patch(Base) | both patch applied | % Improvement |
> > > ------------------------------------------------------------------------|
> > > SMT=off | 16m 13.956s | 6m 18.435s | +61.14 % |
> > > SMT=on | 12m 0.982s | 5m 59.576s | +50.10 % |
> > >
> > > When I tested the below patch independently, I did not observe any
> > > improvements for either smt=on or smt=off. However, in the smt=off scenario,
> > > I encountered hung task splats (with call traces), where some threads were
> > > blocked on cpus_read_lock. Please also refer to the attached call trace
> > > below.
> > > Patch 1:
> > > https://lore.kernel.org/all/20260218083915.660252-4-vishalc@xxxxxxxxxxxxx/
> > >
> > > SMT Mode | Without Patch(Base) | just patch 1 applied | % Improvement
> > > |
> > > ----------------------------------------------------------------------------|
> > > SMT=off | 16m 13.956s | 16m 9.793s | +0.43 %
> > > |
> > > SMT=on | 12m 0.982s | 12m 19.494s | -2.57 %
> > > |
> > >
> > >
> > > Call traces:
> > > 12377] [ T8746] Tainted: G E 7.0.0-rc1-150700.51-default-dirty #1
> > > [ 1477.612384] [ T8746] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> > > disables this message.
> > > [ 1477.612389] [ T8746] task:systemd state:D stack:0 pid:1 tgid:1
> > > ppid:0 task_flags:0x400100 flags:0x00040000
> > > [ 1477.612397] [ T8746] Call Trace:
> > > [ 1477.612399] [ T8746] [c00000000cc0f4f0] [0000000000100000] 0x100000
> > > (unreliable)
> > > [ 1477.612416] [ T8746] [c00000000cc0f6a0] [c00000000001fe5c]
> > > __switch_to+0x1dc/0x290
> > > [ 1477.612425] [ T8746] [c00000000cc0f6f0] [c0000000012598ac]
> > > __schedule+0x40c/0x1a70
> > > [ 1477.612433] [ T8746] [c00000000cc0f840] [c00000000125af58]
> > > schedule+0x48/0x1a0
> > > [ 1477.612439] [ T8746] [c00000000cc0f870] [c0000000002e27b8]
> > > percpu_rwsem_wait+0x198/0x200
> > > [ 1477.612445] [ T8746] [c00000000cc0f8f0] [c000000001262930]
> > > __percpu_down_read+0xb0/0x210
> > > [ 1477.612449] [ T8746] [c00000000cc0f930] [c00000000022f400]
> > > cpus_read_lock+0xc0/0xd0
> > > [ 1477.612456] [ T8746] [c00000000cc0f950] [c0000000003a6398]
> > > cgroup_procs_write_start+0x328/0x410
> > > [ 1477.612462] [ T8746] [c00000000cc0fa00] [c0000000003a9620]
> > > __cgroup_procs_write+0x70/0x2c0
> > > [ 1477.612468] [ T8746] [c00000000cc0fac0] [c0000000003a98e8]
> > > cgroup_procs_write+0x28/0x50
> > > [ 1477.612473] [ T8746] [c00000000cc0faf0] [c0000000003a1624]
> > > cgroup_file_write+0xb4/0x240
> > > [ 1477.612478] [ T8746] [c00000000cc0fb50] [c000000000853ba8]
> > > kernfs_fop_write_iter+0x1a8/0x2a0
> > > [ 1477.612485] [ T8746] [c00000000cc0fba0] [c000000000733d5c]
> > > vfs_write+0x27c/0x540
> > > [ 1477.612491] [ T8746] [c00000000cc0fc50] [c000000000734350]
> > > ksys_write+0x80/0x150
> > > [ 1477.612495] [ T8746] [c00000000cc0fca0] [c000000000032898]
> > > system_call_exception+0x148/0x320
> > > [ 1477.612500] [ T8746] [c00000000cc0fe50] [c00000000000d6a0]
> > > system_call_common+0x160/0x2c4
> > > [ 1477.612506] [ T8746] ---- interrupt: c00 at 0x7fffa8f73df4
> > > [ 1477.612509] [ T8746] NIP: 00007fffa8f73df4 LR: 00007fffa8eb6144 CTR:
> > > 0000000000000000
> > > [ 1477.612512] [ T8746] REGS: c00000000cc0fe80 TRAP: 0c00 Tainted: G
> > > E (7.0.0-rc1-150700.51-default-dirty)
> > > [ 1477.612515] [ T8746] MSR: 800000000000d033 <SF,EE,PR,ME,IR,DR,RI,LE> CR:
> > > 28002288 XER: 00000000
> > >
> > >
> >
> > Default timeout is set to 8 mins.
> >
> > $ grep . /proc/sys/kernel/hung_task_timeout_secs
> > /proc/sys/kernel/hung_task_timeout_secs:480
> >
> > Now that cpus_write_lock is taken once, and SMT mode switch can take
> > tens of minutes to complete and relinquish the lock, threads waiting on
> > cpus_read_lock will be blocked for this entire duration.
> >
> > Although there were no splats observed for "both patch applied" case
> > the issue still remains.
> >
> > regards,
> > vishal